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Prokaryotes
The term prokaryotes applies to two Domains – the Bacteria and
Archaea. Both have simple cellular structure, with no membrane
bound organelles. Archaeans have more characteristics in
common with eukaryotes than with bacteria but like bacteria have
a simple cellular structure. Both groups are ecologically
important. Some bacteria cause diseases in plants and animals. 1
Prokaryotes were likely the first forms of life on Earth.
Fossilized inclusions in rock dating to 3.8 billion years ago
resemble some prokaryotes alive today in bacterial communiiteis
called stromatolites.
Living things accumulate carbon
isotopes differentially. Analysis of
carbon in rock containing likely early
cells have life’s carbon fingerprint.
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Prokaryotes are the most abundant
forms of life. They live in virtually
every habitat on Earth and many
live deep beneath the Earth’s
surface. The diversity of
k
i immense
i
f
prokaryotes
is
andd far
from fully described. It is likely
that less than 10% of bacterial
species have been discovered.
Archaeans live in some of the most extreme environments on
Earth and many are referred to as extremophiles. (See Tree of
Life lecture.) This may be the product of their evolution in the
extreme conditions that were likely common early in Earth’s
history.
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Although the Bacteria and Archaea are very different in many ways
they share many features
• Unicellularity – most exist are single independent cells, but some
can form filaments.
• Cell size – most are much smaller (< 1 μm) than eukaryotic cells
(>10 μm)
• Genetic material – most have a single circular DNA molecule
located in a region of the cell called the nucleoid. The nucleoid
does not have a membrane like a eukaryotic nucleus.
• Cell division is by binary fission. This mode of reproduction is
asexual but they do have several mechanisms for gene transfer
between cells.
g
– they
y are not compartmentalized.
p
Enzyme
y systems
y
• Organization
are commonly associated with the plasma membrane.
• Flagella – are simpler than those of eukaryotes with a single central
fiber and move via a rotary motor at their base.
• Metabolic diversity – they can do photosynthesis in a diversity of
ways and some can utilize inorganic compounds as a source of
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energy.
2
Archaens and Bacteria differ in many ways:
• Plasma membranes – Archaeans have membranes composed of
lipids that are not found in other organisms
Archaeans have ether bonds
b t
l
l andd fatty
f tt
between
glycerol
acids.
Ether bound lipids can be
g
p
linked together
to form a lipid
monolayer with polar heads on
both ends of the lipid.
5
Archaens and Bacteria differ in many ways:
• Cell walls – Bacterial have cell walls containing peptidoglycan
– carbohydrates arranged in a mesh and linked together by
amino acids. Archaeans can have cell walls composed of a
similar
i il compoundd (pseudomurein)
(
d
i ) but
b do
d not have
h
peptidoglycan.
• DNA replication – both have a single origin of replication but
the process in Archaeans is more similar to the DNA replication
of Eukaryotes.
• Gene expression – Bacteria have a single RNA polymerase
used for all transcription.
transcription Archaeans have multiple RNA
polymerases that are more similar to those of Eukaryotes.
Archaeans have introns within some genes. Bacteria do not
have introns.
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3
Analysis of DNA sequences of a diversity of prokaryotes and
eukaryotes suggests that Archaeans share a more recent common
ancestor with Eukaryotes than with Bacteria.
In spite of many similarities and both being called
prokaryotes, Bacteria and Archaeans are not closely related.
They diverged very early in the history of life.
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Molecular data have provided an estimate of the evolutionary
history and relationships of prokaryotes. Prokaryotes are highly
diverse.
The diversity among prokaryotes includes size, shape, ecology,
and biochemistry.
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4
Within all bacterial diversity there are three basic cell shapes
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Bacteria have two different types of cell walls that
can be differentiated using the Gram Stain.
Gram-positive bacteria have a thick
peptidoglycan layer in their cell wall
and stain darkly. Some antibiotics like
penicillin easily kill Gram+ cells by
interfering with cell wall formation.
Gram-negative bacterial have a thin
peptidoglycan
layer
coveredd with
tid l
l
ith a
layer of lipopolysaccharides. They
do not stain darkly. They are also
resistant to the antibiotics that
interfere with cell wall formation.
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5
Many bacteria have one or more
flagella. In bacterial flagella there is a
single central protein filament made of
a protein called flagellin. Flagella
move by spinning which produces a
corkscrew like motion in the filament.
The spinning is powered by a rotary
motor powered by a proton gradient
(similar to ATP synthase).
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In Eukaryotes, respiration depends on
creating proton gradients across the
inner mitochondrial membrane.
in Eukaryotes
also
Photosynthesis
y
y
depends on creating a proton gradient
across the thylakoid membranes of
chloroplasts.
Prokaryotes create proton gradients on
either side of a highly folded plasma
membrane. This allows for respiration
and photosynthesis without specialized
membrane-enclosed organelles.
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6
Sex is the combination of genetic information from two different
sources. In Eukaryotes, sex is usually associated with gamete
formation and creation of new generations. In Bacteria, sex
involves incorporation of new genetic information into a cell.
There are at least three different ways this can occur in different
bacteria:
• Conjugation – many bacteria have an extra small circular DNA
molecule called a plasmid that can be transferred between cells
• Transduction – cellular DNA becomes associated with a phage
and gets transferred between cells
• Transformation – the uptake of DNA directly from the
environment
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Conjugation occurs between cells
with and cells without an F
plasmid. F+ cells contain a
plasmid that encodes the genes
that promote its transfer between
cells. One set of genes encodes
proteins that form a pilus, or tube,
between F+ cells and F- cells. The
completed tube is called a
conjugation bridge.
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7
F plasmids use a special mode of DNA replication called rolling
circle replication to transfer their code to F- cells.
F plasmids are not necessary for cells to live but they often
carry genes that provide antibiotic resistance to cells.
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Plasmids can insert themselves into the cell’s
genome as insertion sequences (IS) by
recombination. Recombination involves the
same mechanisms that eukaryotes use for
and recombination in meiosis.
crossing-over
g
Incorporated plasmids can result in the transfer of
large portions of the cell’s genome between cells
during conjugation by rolling circle replication of
the entire genome.
Plasmids can also use recombination for
excision.
Errors
in
ii
E
i excision
i i can result
l in
i the
h
plasmid containing genes from the cell’s genome.
Plasmids containing portions of the cell’s
genome can also be transferred by conjugation.
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8
Transduction – the transfer of cellular
DNA by viruses
In generalized transduction cellular DNA
accidentally gets packaged into a viral
id When
Wh the
th capsid
id coatt attaches
tt h to
t
capside.
another cell it can then transfer the
original cell’s DNA.
Specialized transduction occurs with
temperate phages (those with a lysogenic
life cycle). An error in the excision of a
lysogenic phage can result in a piece of
the cell’s DNA becoming incorporated
into the phage genome and then packaged
into a capsid. (See lecture on viruses.)
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Transformation – the uptake of DNA from the environment and
incorporation into the cell’s genome.
Conjugation, transduction, and transformation can result in the
transfer
between
bacterial
t
f off antibiotic
tibi ti resistance
it
b t
b t i l cells
ll off the
th
same species or in some cases between different species. This
can result in bacteria having resistance genes to antibiotics to
which they have never been exposed.
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9
Prokaryotes are metabolically diverse. There are four ways that
prokaryotes can acquire energy:
1. Photoautotrophs capture energy from sunlight – photosynthesis.
Cyanobacteria capture light energy with chlorophyll a and use H20
as a source of electrons and produce O2 as a byproduct.
Purple and green sulfur bacteria capture light energy using
bacteriochlorophyll and use H2S as a source of electrons and
leave S as a byproduct.
Some Archaeans are also photosynthetic and use
bacteriorhodopsin or proteorhodopsin to capture light energy.
The energy is used to create a proton gradient to be used for
ATP formation.
2. Chemolithotrophs obtain energy by oxidizing inorganic
compounds. Nitrifiers oxidize ammonia or nitrite. Others oxidize
other substances such as sulfur or hydrogen. Deep sea ocean vent
communities depend on bacteria that capture energy from sulfur
as it escapes from deep in the Earth through the vents.
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3. Photoheterotrophs are the purple and green nonsulfur
bacteria. These capture energy from sunlight but instead of
trapping CO2 as a source of carbon they use carbohydrates or
alcohols
l h l produced
d d by
b other
th organisms
i
as a carbon
b source.
4. Chemoheterotrophs are the most common bacteria. They
obtain energy by using compounds produced by other
organisms. They can be decomposers of organic material.
They can also be pathogens and feed directly on other living
things.
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10
Bacteria are pathogens of plants and animals. The development of
antibiotics and immunization has helped to decrease the damage
caused by bacteria. Before antibiotics and immunization, one in
five children in the United States died of a bacterial disease before
the age of five. Bacterial diseases of children and adults include
cholera, leprosy, tetanus, pneumonia, whooping cough, diphtheria,
and tuberculosis.
Bacteria cause tooth decay by producing acids that erode tooth
enamel. Sugars are easily metabolized by such bacteria and are
thus associated with tooth decay.
Bacteria are one cause of ulcers. They cause lesions and bleeding
of the lining of the stomach.
Many STDs are caused by bacteria. Syphilis, chlamydia, and
gonorrhea are spread by sexual contact and cause many secondary
health problems such as nerve damage and heart disease.
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Some prokaryotes are ecologically important and beneficial
•Decomposition returns the chemicals trapped in one living
organisms to the ecosystem
•Fixation captures important chemicals and incorporates them into
important biological molecules.
C b is
Carbon
i fixed
fi d by
b photosynthetic
h
h i prokaryotes
k
Nitrogen as N2 gas is abundant but is not useful to living
things. N2 can be captured by bacteria as ammonia (NH3).
Ammonia can be used to make amino acids and
nitrogenous bases. Conversion of ammonia to nitrates
makes nitrogen available to plants.
•Bacteria have beneficial symbiotic relationships with many other
organisms.
Nitrogen fixers live in association with the roots of some plants
and make N available to those plants.
Bacteria live within the guts of most animals and help with
digestion and provide some essential vitamins to their hosts.23
Bacteria have become increasing important economically
• Genetically engineered bacteria are being used to produce
many beneficial products such as hormones, enzymes,
vitamins antibiotics,
vitamins,
antibiotics and other pharmaceutical products.
products
• Bacteria are bioremediators. They naturally degrade many
chemicals that eukaryotes can’t and many are now being used
to clean up human sewage, the waste of other organisms, and
toxic chemicals produced by industries.
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